Fasted cardio is when you do cardio on an empty stomach. It works for some lifestyles but not others. Keep reading to learn about the benefits and potential risks before deciding if it’s right for you.
Fasted cardio is performed when your body is in a fasted state. Being in a fasted state means your body has already used the calories from your previous meal and is relying on stored calories for energy. Basically, it means doing cardio on an empty stomach.
This would normally happen first thing in the morning, after sleeping overnight. But it can also happen later in the day if you practice intermittent fasting.
Compared to cardio without the fasting aspect, fasted cardio is touted as a way to accelerate fat loss. While that may sound great, its effectiveness hasn’t been fully proven. Let’s dive deeper.
If you’re generally healthy, it’s fine to incorporate short or moderate-length steady-state fasted cardio sessions into your routine.
However, if you’ll be exercising for any extended period of time or doing a high intensity workout, fasted cardio can be risky due to potential side effects of low blood sugar or dehydration. These include:
- lightheadedness
- dizziness
- shaking
- passing out
The idea behind fasted cardio is that if you fast overnight and work out first thing in the morning, your body is depleted of glucose — its main source of energy — and will instead use stored fat for fuel.
Research is mixed on the effectiveness of this approach.
One review found that fasted exercise led to higher rates of fat oxidation during low-to-moderate intensity workouts. However, the difference in fat oxidation between fasted and fed states decreased in more intense exercises.
Additionally, this review mainly consisted of moderately-trained males.
While more research is needed to make more concrete claims, fasted cardio has some other potential benefits:
- If you’re tight on time, fasted cardio saves you from having to prepare, eat, and digest a meal beforehand.
- If you practice intermittent fasting, fasted cardio allows you to exercise before you eat for the day.
- If you prefer working out on an empty stomach, fasted cardio could be an effective option, especially if you have a sensitive stomach or feel more energetic without a meal before a workout.
The most important aspect of weight loss is burning more calories than you consume. Research is mixed on whether fasted cardio actually promotes fat loss.
In one study, 8 men and 8 women were split into fasting and non-fasting groups. Both groups consumed breakfast and lunch. Two hours before exercising, the non-fasting group consumed a meal. Both groups completed 30 minutes of steady-state cycling and a 15-minute performance test.
The study found that the fasting group had a higher rate of fat oxidation and lower rates of carbohydrate oxidation. However, performance, energy, motivation, and enjoyment were lower in the fasting group than that of the non-fasting group.
In general, if you’re looking to lose weight, increasing your daily movement, whether fasted or not, is still the best plan.
Although fasted cardio does have some benefits, it’s important to be aware of potential risks.
It could hinder muscle-building
Research has found that if there are not enough carbohydrates in your system for energy, your body begins a process called gluconeogenesis, which converts protein into fuel. This means there’s less protein left to rebuild muscle.
It may hinder performance
Especially if you’re knocking out a moderate or high intensity workout — like HIIT, boot camp, or weight training — on an empty stomach, your energy levels will not be up to par without some fuel beforehand.
Also, look out for signs of low blood sugar and dehydration.
Avoid fasted cardio if you have a medical condition that’s affected by low blood sugar or blood pressure or if you’re pregnant.
It’s also best for those beginning at exercise to shy away from fasted cardio — understanding your body well should be the first step on your exercise journey.
If you’re generally healthy, the decision to incorporate fasted cardio is a personal one. If you’ve never tried it before, start slowly.
First, make sure to hydrate before and during the session.
Try a low to moderate intensity steady-state session — like walking, running, biking, or the elliptical — for 10 minutes and see how you feel. If it goes well, work your way up to 30 minutes as time goes on.
Afterward, make sure to fuel up with a balanced meal or snack packed with protein and carbs.
Avoid high intensity work — where your heart is pumping hard — or any session more than an hour long during fasted cardio.
You can incorporate low intensity steady-state fasted cardio on multiple days throughout the week, but make sure you’re taking 1 or 2 rest days too.
Cardio, fasted or not, is great for your body. And while nutrition is key for weight loss, cardio can help you reach a weight loss goal.
While research on its metabolic effects is still inconclusive, fasted cardio may work better for your lifestyle or preferences, so if you’re generally healthy, feel free to give it a go.
